1. Technical Field
The present invention relates to a program shutter to perform an opening and closing operation of a shutter blade using a step motor.
2. Prior Art
A program shutter to open and close a shutter blade using a step motor is so constituted that drive pulses having a definite period are applied to a step motor during a release operation so that the motor is rotated in the forward direction and a shutter sector is opened gradually, and when the step rotation corresponding to the desired exposure quantity has been performed, the phase of the drive pulses is changed so that the step motor is rotated in the reverse direction and the shutter sector is closed so as to obtain prescribed exposure.
Since the maximum rotation speed of the step motor used for the shutter is as low as about 500-1000 pulses/sec, the minimum time required for the rotation of one step is restricted, and particularly when exposing for a short time, a large error may be produced and the most favorable exposure step cannot be obtained. This has become a serious problem in view of the recent development of high sensitivity films.
In order to solve such as problem, the applicant of the present invention has already proposed a program shutter where the rotation step number of a step motor and an interpolation quantity are determined corresponding to a certain exposure quantity so that when the shutter is opened, a sector is driven in step according to step drive pulses corresponding to the rotation step number, and when the last step drive pulse is outputted and thereafter a period of time corresponding to the interpolation quantity elapses, the step motor is rotated in the reverse direction and the interval between steps is subdivided so as to obtain the suitable exposure quantity. In such a program shutter, however, since considerable data processing is required for the timing control, in random logic circuitry comprised of combinations of logical elements such as gates, flipflops and counters, regularity between element arrangement and connection is obstructed and, therefore, not only is the circuit complicated but also the system is fixed so that apparatus difference accompanied with such an electric shutter cannot be corrected by circuit means.